The present invention relates to spinel-type spherical black iron oxide particles having a large magnetization in an external magnetic field of 1 kOe and as low a coercive force as possible so as to have a small magnetic cohesion, and a process for producing such spinel-type spherical black iron oxide particles.
Spinel-type spherical black iron oxide particles according to the present invention are mainly used as material particles for magnetic toners, magnetic carriers, magnetic cards, etc.
Since spinel-type black iron oxide particles are ferromagnetic particles, they are mixed with a resin so as to produce composite particles, which are used as material particles for magnetic toners and magnetic carriers.
Spinel-type black iron oxide particles are sometimes dispersed in a vehicle so as to obtain a magnetic coating. The thus-obtained magnetic coating is directly applied to a card substrate or a magnetic tape obtained by applying the magnetic coating to a base film is adhered to a card substrate. In this way, spinel-type black iron oxide particles are also used as material particles for magnetic cards such as credit cards provided with a magnetic stripe, railway tickets, railway season tickets, highway passes, telephone cards and railway cards.
In any of the above-described fields, there is no end to demand for higher capacity and higher quality of the products, and spinel-type black iron oxide particles are required to have improved properties, in particular, to have as large an initial magnetization as possible and excellent dispersibility which facilitates the kneading of the particles with a resin or the mixing thereof with a vehicle.
In order to increase the initial magnetization, it is necessary that the magnetization in an external magnetic field of 1 kOe is as large as possible.
In order to increase the dispersibility, it is necessary that the coercive force is as low as possible so as to have a small magnetic cohesion.
This fact is described with respect to spinel-type black iron oxide particles for magnetic toners in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 (1980), "Magnetic powders for magnetic toners of a one-component type are generally required to have the following properties. I) The magnetic flux density in a magnetic field of about 10.sup.3 Oe is as high as possible. For example, in an external magnetic field of 1,000 Oe, it is necessary that the maximum magnetization (.sigma..sub.m) is about not less than 40 emu/g. This is in order to keep the height of the spike as a magnetic brush high . . . IV) Magnetic powders are well mixed with a resin. The particle size of an ordinary toner is not more than several 10 .mu.m, so that the microscopic mixing degree in the toner is important for the properties of the toner.
With respect to spinel-type black iron oxide particles for magnetic carriers, this fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 63-33754 (1988), "Since a magnetic resin carrier is a mixture of a magnetic powder and a binder resin which is a nonmagnetic material, the magnetic flux density (.sigma..sub.s) is lower than that of a carrier consisting of a magnetic material, so that the magnetic resin carrier is apt to cause bead carry-out in the development, . . . " and "A magnetic resin carrier for electrophotography according to the present invention, has a larger magnetic flux density in an external magnetic field of 1 kOe than a magnetic flux density obtained by a conventional magnetic powder, and the picture quality thereof is excellent, free from bead carry-out", and in Japanese Patent Application Laid-Open (KOKAI) No. 61-53660 (1986), "According to the present invention, . . . the dispersibility of primary particles is improved and the magnetic powder is uniformly dispersed, and the difference in the static property and the magnetic characteristics between magnetic developer particles is reduced".
As to the spinel-type black iron oxide particles as magnetic particles for magnetic cards, this fact is described in Table 2: Properties Required of Magnetic Coated Film and Their Controlling Factors in Magnetic Cards and Chemical Technique at pp. 68 to 72 of Journal of Chemical Technique, MOL (1985), published by Ohm-sha, "Required properties 1. High sensitivity, large reproducing output, large saturation, residual magnetic flux density, sharp rising of the initial magnetization curve, . . . "and" . . . the properties are greatly changed with the state of dispersion of a magnetic material . . . . It is important to have a good dispersion system".
Spinel-type black iron oxide particles as material particles for magnetic toners, magnetic carriers and magnetic cards, are generally produced directly from an aqueous solution, namely, by "wet process", because this process makes easy to produce discrete particles, in other words, particles having good dispersibility.
The wet process comprises the steps of: blowing an oxidizing gas in a temperature range of 60.degree. to 100.degree. C. into an aqueous solution which contains an Fe-containing precipitate such as Fe(OH).sub.2 and FeCO.sub.3, and which is obtained by mixing an aqueous ferrous salt solution such as ferrous sulfate and an aqueous alkaline solution such as sodium hydroxide and sodium carbonate thereby obtaining a black precipitate, removing the acid radical and the like by washing the precipitate with water, and drying the precipitate so as not to cause color change (Japanese Patent Publication No. 44-668 (1969)).
As a method of improving various properties of the spinel-type iron oxide particles obtained by the wet process, a method of adding a zinc compound to the wet reaction system, thereby obtaining zinc-containing spinel-type iron oxide particles (Japanese Patent Publications No. 42-20381 (1967), and No. 59-43408 (1984), Japanese Patent Application Laid-Opens (KOKAI) No. 56-5330 (1981), No. 61-86424 (1986), No. 62-91423 (1987), and a method of heating such zinc-containing spinel-type iron oxide particles at 300.degree. to 700.degree. C. (Japanese Patent Application Laid-Open (KOKAI) No. 63-68847) are known.
Spinel-type black iron oxide particles having a large magnetization in an external magnetic field of 1 kOe and as low a coercive force as possible so as to have a small magnetic cohesion are in the strongest demand at present, but the spinel-type iron oxide particles obtained in the above-described methods cannot be said to satisfy both properties described above.
As the result of the present inventor's studies, it has been found that spinel-type spherical black iron oxide particles obtained by preparing a mixed suspension containing ferrous hydroxide and zinc hydroxide having a pH of 6 to 9 in a non-oxidizing atmosphere by using an aqueous ferrous salt solution, a zinc compound and an aqueous alkaline solution, and blowing an oxygen-containing gas into the mixed suspension so as to oxidize ferrous hydroxide so that the pH thereof is in the range of not less than 4 and less than 6 when an oxidization degree is such that Fe.sup.3 + in the mixed suspension is 60 to 70 mol % based on the total amount of iron and zinc in the oxidization step (this case will be referred to as "the oxidization degree is 60 to 70 mol %" hereinunder) have a large magnetization in an external magnetic field of 1 kOe and as low a coercive force as possible. The present invention has been achieved on the basis of this finding.